1. Field of the Invention This invention is a quantum well transistor which uses the resonant tunnel effect and possesses a low base current and high gain in the microwave range (several tens of gigahertz).
2. Discussion of the Background
In a semiconductor device, a quantum well is formed by a double barrier, or double heterojunction structure--consequently a structure in the body of the device--in which the well layer is very fine (1-20 nm) and has a narrow forbidden band, the two barrier layers having a wide forbidden band. For charge carriers, for example, conduction electrons, the potential energy configuration forms a well. The most favorable energy levels occur when the electrons are in the well. The well layer itself is sufficiently thin to generate quantum phenomena, hence the name "quantum well".
It is known that filtering an electron flow in quantum well structures between two ohmic contacts gives highly non-linear current-voltage characteristics. The most spectacular example is the double-barrier resonant tunnel effect diode which has a negative differential resistance coefficient at ambient temperature. Active non-linear components offering a fast response are important in analog microwave applications and ultra-fast multi-state digital applications.
Although constructive interference effects based on resonant effects can be attained in a simple diode, a third control electrode facilitates the development of applications. It would appear preferable to apply control directly in the well: this will allow the development of components in which physical phenomena occur virtually instantaneously and which are highly sensitive to voltage.
Several types of quantum well transistors are known to the prior art, including some which use a resonant tunnel effect to allow carrier injection. In these transistors, the layer of semiconductor material which forms the quantum well is not bounded laterally, i.e. in the direction in which the material grows, and the layer spreads under the lateral metallizations which form the base (if the structure is vertical). Charge carriers injected by the tunnel effect from the emitter through the first barrier and the base are only collected if they have sufficient energy to cross the second barrier in the quantum well at the collector level; other charge carriers become thermal in the base and dissipate their energy.
Consequently, while certain high-energy electrons can cross the barrier to the collector, other electrons, with lower energy levels, remain in the well, since they are incapable of crossing the second barrier; they escape laterally via the base metallization, increasing the base current and reducing the transistor gain.